Re: [Qemu-devel] [PATCH] block: add a knob to disable multiwrite_merge

[Max Reitz <mreitz@redhat.com>]

On 20.10.2014 at 15:19, Peter Lieven wrote:
> On 20.10.2014 15:15, Max Reitz wrote:
>> On 20.10.2014 at 14:48, Peter Lieven wrote:
>>> On 20.10.2014 14:19, Max Reitz wrote:
>>>> On 2014-10-20 at 14:16, Peter Lieven wrote:
>>>>> On 20.10.2014 13:51, Max Reitz wrote:
>>>>>> On 2014-10-20 at 12:03, Peter Lieven wrote:
>>>>>> [...]
>>>>>
>>>>> Can you further help here. I think my problem was that I don't 
>>>>> have access to the commandline options in bdrv_open?!
>>>>
>>>> You do. It's the "options" QDict. :-)
>>>
>>> Maybe I just don't get it.
>>>
>>> If I specify
>>>
>>> qemu -drive if=virtio,file=image.qcow2,write-merging=off
>>>
>>> and check with
>>>
>>> qdict_get_try_bool(options, "write-merging", true);
>>>
>>> in bdrv_open() directly before bdrv_swap I always get true.
>>
>> Hm, judging from fprintf(stderr, "%s\n", 
>> qstring_get_str(qobject_to_json_pretty(QOBJECT(options))));, it's 
>> there for me (directly after qdict_del(options, "node-name"). The 
>> output is:
>>
>> Qemu wrote:
>>> {
>>>     "filename": "image.qcow2"
>>> }
>>> {
>>>     "write-merging": "off"
>>> }
>>> qemu-system-x86_64: -drive 
>>> if=virtio,file=image.qcow2,write-merging=off: could not open disk 
>>> image image.qcow2: Block format 'qcow2' used by device 'virtio0' 
>>> doesn't support the option 'write-merging'
>>
>> But as you can see, it's a string and not a bool. So the problem is 
>> that there are (at least) two parameter "types" in qemu: One is just 
>> giving a QDict, and the other are QemuOpts. QDicts are just the raw 
>> user input and the user can only input strings, so everything is just 
>> a string. As far as I know, typing everything correctly is done by 
>> converting the QDict to a QemuOpts object (as you can see in 
>> generally every block driver which supports some options (e.g. qcow2) 
>> and also in blockdev_init(), it's qemu_opts_absorb_qdict()).
>>
>> Sooo, right, I forgot that. Currently, there are no non-string 
>> non-block-driver-specific options for mid-tree BDS (in contrast to 
>> the root BDS, which are parsed in blockdev_init()), so you now have 
>> the honorable task of introducing such a QemuOptsList along with 
>> qemu_opts_absorb_qdict() and everything to bdrv_open_common(). *cough*
>
> I would appreciate if someone with better knowledge of this whole 
> stuff would start this. Or we postpone this know until all the ongoing 
> conversions are done.

I can try and create some barebone which your patches can then be based 
on. I probably don't have the knowledge either, but I'm daring enough to 
do it anyway. ;-)

Max